The invention relates to internal combustion engines, particularly of the rotary type such as disclosed in U.S. Pat. No. 2,988,065 granted June 13, 1961 to Wankel et al and more particularly to stratified-charge rotary combustion engines having high pressure fuel injection such as disclosed in U.S. Pat. No. 3,246,636 granted Apr. 19, 1966 to Bentele and U.S. Pat. No. 3,894,518 granted July 15, 1975 to Gavrun et al and which are capable of operating as a spark-ignition engine on diesel-type (low octane) fuels. Such stratified charge rotary engines have an unthrottled air intake and therefore operate with a large quantity of excess air. As a result, the Bentele and Gavrun et al stratified charge rotary engines have high thermal efficiencies and low hydrocarbon exhaust emissions. This is particularly true at engine power outputs below the high power range of the engine. Thus, when such an engine is operated, for example, at full power, the fuel must be distributed over the entire combustion chamber in order for all of the fuel to mix with air so as to achieve complete combustion. Hence, at high engine power outputs of such a stratified charge engine, the resulting combustion is essentially as in a carbureted engine with its attendant lower thermal efficiencies and high exhaust emissions. At lower engine powers, however, the fuel can be confined to and burn in a mid-portion of each combustion chamber where it is substantially enveloped by the excess air whereby complete combustion is achieved while at the same time this enveloping excess air minimizes combustion heat loss to the combustion chamber walls thereby providing high thermal efficiencies. Also, in such stratified charge rotary engines, particularly of the type shown in the Bentele patent, difficulty has been experienced in obtaining completely acceptable ignition firing regularity over the entire operating range of the engine.
Diesel-type operation of an internal combustion engine requires compression ratios of the order of magnitude of 15:1. Compression ratios of this magnitude are difficult to achieve in rotary engines of the type disclosed in the aforementioned patents.
As described in said Wankel et al patent, such engines have a multi-lobe cavity which preferably has an epitrochoidal profile. The shape of this epitrochoidal engine cavity determines the compression ratio. Thus, an epitrochoid having a smaller ratio of a/b has a larger compression ratio, where a is equal to one-half the length of the major axis of the epitrochoid and b is equal to one-half the length of its minor axis. Today it is more common to describe the shape of this epitrochoid in terms of a so-called "K" factor which is equal to the ratio of R/e where R is the radial distance from the center of the rotor to the tip of its apex seals and e is the distance between the rotor center and engine axis. In general, the magnitude of the "K" factor increases as the ratio a/b decreases. Hence, for higher compression, a rotary engine of the type shown in said Wankel et al patent should have a high "K" factor.
As is evident from the disclosure of said Wankel et al patent, at high compression ratios, the shape of each engine working chamber at its top dead center position becomes extremely thin in a radial direction and, therefore, combustion in the working chambers is subject to severe cooling or chilling by the walls of the chamber. For this reason it is difficult to make a successful diesel-type rotary combustion engine of the type disclosed in the Wankel et al patent simply by changing the engine "K" factor to increase the engine compression ratio, for example, to about 15:1.
Diesel-type rotary combustion engines have been designed with a supercharger for providing the necessary pressures for compression ignition, that is, diesel operation, whereby the volumetric compression ratio of the main engine (apart from the supercharger) can be substantially less than that required for diesel operation. U.S. Pat. No. 3,228,183 granted Jan. 11, 1966 to Feller and U.S. Pat. No. 3,405,692 granted Oct. 15, 1968 to Paschke are examples of such a compound diesel engine. In both these prior art diesel engines the supercharger is a positive-displacement type supercharger which is driven from the engine shaft. With such a compound engine, the overall volumetric compression ratio of the engine is essentially the same throughout the operating range of the engine. Such a compund engine can therefore operate as a diesel throughout the operating range of the engine. For example, if the compression ratio of the supercharger is made equal to 2:1 and that of the main engine is made equal to 8:1, then the overall compression ratio becomes equal to 16:1 which is sufficient for diesel operation.
In order to further improve the efficiency of these prior art diesel engines, they are also provided with an expander to utilize some of the energy otherwise lost in the engine exhaust gases. If, as in the Feller patent (FIG. 1) or Paschke (FIG. 3), the supercharger and expander are combined into a common unit, then the relative positions of the porting connections of the supercharger and expander with the main engine unit involve compromises which necessarily reduce the overall engine efficiency. On the other hand, if separate compressor and expander units are used with each drivably connected to the engine shaft, as it also shown in the Feller (FIG. 10) and Paschke (FIG. 6) patents, then the complexity and size of the compound engine are increased as a result of using two such units, each with its geared connection to the engine shaft.